1. Field of the Invention
This invention relates to an apparatus for measuring blood flow, and more particularly to an apparatus for noninvasively obtaining blood flow information within in vivo tissue while providing the capability to superimpose the blood flow information on a video image of the tissue to obtain a composite visible image.
2. Description of the Prior Art
Laser Doppler apparatuses or laser speckle apparatuses have ben marketed as apparatuses for noninvasively obtaining information on blood flow. These apparatuses measure blood flow factors by directing a laser beam into the blood steam, picking up light scattered by erythrocytes moving in the blood, and analyzing the frequency spectrum of the received light for determining the frequency gradient. An apparatus based on this method is disclosed, for example, in Japanese Patent Public Disclosure Sho 60(1985)-203235.
Such conventional apparatuses use a laser beam projecting probe and a light receiving probe, both constituted of optical fibers, and the measurement depths within the in vivo tissue is regulated by adjusting the distance between the probes (See Fujii et al., Measurement of blood flow in skin using laser beam phenomenon (V) (Japan Laser Medical Magazine), Vol. 6, No. 3 (January 1986)).
An example of the arrangement used is shown in FIG. 4. A laser beam is directed into in vivo tissue P from an optical fiber F1 and the scattered light is received by an optical fiber F2. The intensity of the light received by the light receiving fiber F2 is governed by its distance from the beam projecting fiber F1.
Assuming the tissue to be a perfect light scattering body, Fujii et al. approximated the intensity of the light received by the light receiving fiber F2 as shown below (Fujii et al., Evaluation of skin blood flow using laser speckle phenomena (VII) (The Journal of Japan Society for Laser Medicine), Vol. 7, No. 3 (January 187)). EQU Im=I.sub.0 Exp {-.gamma.(R1+r2)} (1)
where
Im: intensity of received light PA1 I.sub.0 : intensity of irradiating beam PA1 .gamma.: coefficient of attenuation owing to absorption and scattering PA1 r1: distance between end surface P1 of beam projecting fiber and light scattering particles (erythrocytes) PA1 r2: distance between end surface P2 of light receiving fiber and light scattering particles (erythrocytes)
This equation being that of an ellipse having its foci at the points P1 and P2 where the light enters and leaves the tissue, it can be seen that the length of the light path for which scattered light can be received increases with increasing distance between the points P1 and P2. In other words, scattered light from deeper parts of the tissue can be received by increasing the distance between the optical fibers F1 and F2.
As this conventional arrangement requires the optical fiber probes to be brought in contact with the tissue with respect to which measurement is being conducted, it is apt to have undesirable effects on the patient, such as making him or her feel uneasy or uncomfortable.
Scanning a large measurement region using the conventional arrangement involves the troublesome work of repeatedly repositioning the measurement probes and, moreover, requires the position information to be recorded after each repositioning. The measurement work is thus complicated and laborious.